Suppression of thymic development by the dominant-negative form of Gads

The combined loss of Gads and CD127 reveals a novel function of Gads prior to TCRβ expression

The Gads adaptor protein is an essential component of the T cell signaling complex critical for T cell receptor-mediated calcium mobilization. After expression of TCRβ in T cell precursors, Gads is required for optimal Bcl-2 expression and cell survival. Similarly, the IL-7 receptor chain CD127 is also necessary for optimal Bcl-2 expression and cell survival in TCRβ-expressing thymocytes. Based on these observations, we tested whether Gads and CD127 might regulate convergent or linear signaling pathways by crossing Gads(-/-) mice with CD127(-/-) mice. Thymi from Gads(-/-)CD127(-/-) mice were barely detectable and many of the thymocytes were within the DN1 population. By contrast, B cell development in the Gads(-/-)CD127(-/-) mice was comparable to that of CD127(-/-) mice, indicating that the combined loss of Gads and CD127 did not lead to a global deficit in hematopoiesis. Analysis of Lin(-)Sca-1(+)c-kit(+) bone marrow cells and bone marrow chimera experiments indicated that Gads(-/-)CD127(-/-) T cell precursors either failed to migrate into the thymus or survive in the thymus. These data demonstrate that Gads functions at a stage of T cell development that had not been previously described.

LIME mediates immunological synapse formation through activation of VAV

Lck Interacting Membrane protein (LIME) was previously characterized as a transmembrane adaptor protein mediating TCR-dependent T cell activation. Here, we show that LIME associates with Vav in response to TCR stimulation and is required for Vav guanine nucleotide exchange factor (GEF) activity for Rac1. Consistent with this finding, actin polymerization at the immunological synapse (IS) was markedly enhanced by overexpression of LIME, but was reduced by expression of a LIME shRNA. Moreover, TCR-mediated cell adhesion to ICAM-1, laminin, or fibronectin was downregulated by expression of LIME shRNA. In addition, in the IS, LIME but not LAT was found to localize at the peripheral-supramolecular activation cluster (p-SMAC) where the integrins were previously shown to be localized. Together, these results establish LIME as a transmembrane adaptor protein linking TCR stimulation to IS formation and integrin activation through activation of Vav.

Conditional deletion of SLP-76 in mature T cells abrogates peripheral immune responses

The adaptor protein Src homology 2 domain-containing leukocyte-specific protein of 76 kDa (SLP-76) is central to the organization of intracellular signaling downstream of the T-cell receptor (TCR). Evaluation of its role in mature, primary T cells has been hampered by developmental defects that occur in the absence of WT SLP-76 protein in thymocytes. Here, we show that following tamoxifen-regulated conditional deletion of SLP-76, mature, antigen-inexperienced T cells maintain normal TCR surface expression but fail to transduce TCR-generated signals. Conditionally deficient T cells fail to proliferate in response to antigenic stimulation or a lymphopenic environment. Mice with induced deletion of SLP-76 are resistant to induction of the CD4+ T-cell-mediated autoimmune disease experimental autoimmune encephalomyelitis. Altogether, our findings demonstrate the critical role of SLP-76-mediated signaling in initiating T-cell-directed immune responses both in vitro and in vivo and highlight the ability to analyze signaling processes in mature T cells in the absence of developmental defects.

Gads-deficient thymocytes are blocked at the transitional single positive CD4+ stage

Positive selection of T-cell precursors is the process by which a diverse T-cell repertoire is established. Positive selection begins at the CD4(+)CD8(+) double positive (DP) stage of development and involves at least two steps. First, DP thymocytes down-regulate CD8 to become transitional single positive (TSP) CD4(+) thymocytes. Then, cells are selected to become either mature single positive CD4(+) or mature single positive CD8(+) thymocytes. We sought to define the function of Gads during the two steps of positive selection by analyzing a Gads-deficient mouse line. In Gads(+/+) mice, most TSP CD4(+) thymocytes are TCR(hi)Bcl-2(hi)CD69(+), suggesting that essential steps in positive selection occurred in the DP stage. Despite that Gads(-/-) mice could readily generate TSP CD4(+) thymocytes, many Gads(-/-) TSP CD4(+) cells were TCR(lo)Bcl-2(lo)CD69(-), suggesting that Gads(-/-) cells proceeded to the TSP CD4(+) stage prior to being positively selected. These data suggest that positive selection is not a prerequisite for the differentiation of DP thymocytes into TSP CD4(+) thymocytes. We propose a model in which positive selection and differentiation into the TSP CD4(+) stage are separable events and Gads is only required for positive selection.

In vivo disruption of T cell development by expression of a dominant-negative polypeptide designed to abolish the SLP-76/Gads interaction

Multi-molecular complexes nucleated by adaptor proteins play a central role in signal transduction. In T cells, one central axis consists of the assembly of several signaling proteins linked together by the adaptors linker of activated T cells (LAT), Src homology 2 domain-containing leukocyte-specific phosphoprotein of 76 kDa (SLP-76), and Grb2-related adaptor downstream of Shc (Gads). Each of these adaptors has been shown to be important for normal T cell development, and their proper sub-cellular localization is critical for optimal function in cell lines. We previously demonstrated in Jurkat T cells and a rat basophilic leukemic cell line that expression of a 50-amino acid polypeptide identical to the site on SLP-76 that binds to Gads blocks proper localization of SLP-76 and SLP-76-dependent signaling events. Here we extend these studies to investigate the ability of this polypeptide to inhibit TCR-induced integrin activity in Jurkat cells and to inhibit in vivo thymocyte development and primary T cell function. These data provide evidence for the in vivo function of a dominant-negative peptide based upon the biology of SLP-76 action and suggest the possibility of therapeutic potential of targeting the SLP-76/Gads interaction.

Gads-/- mice reveal functionally distinct subsets of TCRbeta+ CD4-CD8- double-negative thymocytes

TCRbeta expression in CD4(-)CD8(-) double-negative (DN) thymocytes induces signaling pathways that promote survival and proliferation, as well as differentiation into CD4(+)CD8(+) double-positive thymocytes. The signaling pathways that regulate survival, proliferation, and differentiation remain unclear. We used Gads-deficient mice to investigate the signaling pathways that regulate these cell fates. During this investigation, we focused on TCRbeta(+) DN thymocytes and found that there are at least three functionally distinct subsets of TCRbeta(+) DN thymocytes: TCRbeta(+) DN3E, TCRbeta(+) DN3L, and TCRbeta(+) DN4. Survival and proliferation of TCRbeta(+) DN3E were independent of Gads, but survival and proliferation of TCRbeta(+) DN3L cells were Gads dependent. Likewise, expression of Bcl-2 in TCRbeta(+) DN3E cells was Gads independent, but Gads was necessary for Bcl-2 expression in TCRbeta(+) DN3L cells. Bcl-2 expression was not dependent on Gads in TCRbeta(+) DN4 cells, but proliferation of TCRbeta(+) DN4 cells was Gads dependent. Gads was not required for the differentiation of DN thymocytes into DP thymocytes. In fact, Gads(-/-) DN3E cells differentiated into DP thymocytes more readily than wild-type cells. We conclude that signaling pathways required to initiate TCRbeta-induced survival and proliferation are distinct from the pathways that maintain survival and proliferation. Furthermore, signaling pathways that promote survival and proliferation may slow differentiation.

The Gads (GrpL) adaptor protein regulates T cell homeostasis

Little is known about the role of the Gads (GrpL) adaptor protein in mature T cell populations. In this study we show that the effects of Gads deficiency on murine CD4(+) and CD8(+) T cells are markedly different. Gads(-/-) CD4(+) T cells were markedly deficient in the spleen and had an activated phenotype and a rapid turnover rate. When transferred into a wild-type host, Gads(-/-) CD4(+) T cells continued to proliferate at a higher rate than wild-type CD4(+) T cells, demonstrating a defect in homeostatic proliferation. Gads(-/-) CD8(+) T cells had a memory-like phenotype, produced IFN-gamma in response to ex vivo stimulation, and underwent normal homeostatic proliferation in wild-type hosts. Gads(-/-) T cells had defective TCR-mediated calcium responses, but had normal activation of ERK. Gads(-/-) CD4(+) T cells, but not CD8(+) T cells, had a severe block of TCR-mediated proliferation and a high rate of spontaneous cell death and were highly susceptible to CD95-induced apoptosis. This suggests that the rapid turnover of Gads(-/-) CD4(+) T cells is due to a defect in cell survival. The intracellular signaling pathways that regulate homeostasis in CD4(+) and CD8(+) T cells are clearly different, and the Gads adaptor protein is critical for homeostasis of CD4(+) T cells.

The E47 transcription factor negatively regulates CD5 expression during thymocyte development

The expression of CD5 increases progressively as thymocytes mature. We have shown that CD5 expression is controlled by a tissue-specific regulatory promoter located upstream of the CD5 translation start sites. Deletion of this regulatory promoter, which contains three potential transcription factor binding sites (CCAAT, kappa E2, and ets) reduces the promoter activity to basal level. Of these sites, only ets proved essential for CD5 expression in T cell lines. Here, we introduce a role for the E47 transcription factor and the CD5 promoter kappa E2 site in regulating CD5 expression during thymocyte development. Using T cell lines, we show that (i) mutation of the kappa E2 site in the CD5 regulatory promoter results in a significant elevation of CD5 promoter activity; (ii) the E47 transcription factor binds to the kappa E2 site; and (iii) overexpression of E47 inhibits CD5 expression. We then show, in high-dimensional fluorescence-activated cell sorting studies with primary thymocytes at successive developmental stages, that (i) intracellular E47 levels decrease as surface CD5 expression increases; (ii) E47 expression is down-regulated and CD5 expression is correspondingly up-regulated in DN3 thymocytes in RAG-2-deficient mice injected with anti-CD3 to mimic pre-T cell receptor stimulation; and (iii) E47 expression is down-regulated and CD5 expression is up-regulated when double positive thymocytes are stimulated in vitro with anti-CD3. Based on these data, we propose that E47 negatively regulates CD5 expression by interacting with the kappa E2 site in the CD5 regulatory promoter and that decreases in E47 in response to developmental signals are critical to the progressive increase in CD5 expression as thymocytes mature.

Identification of AMSH-LP containing a Jab1/MPN domain metalloenzyme motif

We have isolated a cDNA clone encoding a new AMSH (associated molecule with the SH3 domain of STAM) family protein, termed AMSH-like protein (AMSH-LP). AMSH-LP has similar characteristics to AMSH; both AMSH-LP and AMSH are expressed ubiquitously in various human tissues, contain a putative nuclear localization signal (NLS), an Mpr/Pad1/N-terminal (MPN) domain, and a Jab1/MPN domain metalloenzyme (JAMM) motif in their structures, and are excluded from the nucleus when lacking either the NLS or MPN domain. Moreover, we observed an enhancement of interleukin 2 (IL-2)-mediated c-myc induction in AMSH-LP-transfected cells similar to that seen in AMSH-transfected cells, suggesting a functional similarity between AMSH-LP and AMSH. However, the present study demonstrated that AMSH-LP, unlike AMSH, fails to bind to the SH3 domains of STAM1 (signal transducing adaptor molecule 1) and Grb2. These results suggest that AMSH-LP and AMSH may have different functions.

The Ras/MAPK cascade and the control of positive selection

Immature double positive (DP) thymocytes bearing a T cell receptor (TCR) that interacts with self-major histocompatibility complex (MHC) molecules receive signals that induce either their differentiation (positive selection) or apoptosis (negative selection). Furthermore, those cells that are positively selected develop into two different lineages, CD4 or CD8, depending on whether their TCRs bind to MHC class II or I, respectively. Positive selection therefore involves rescue from the default fate (death), lineage commitment, and progression to the single positive (SP) stage. These are probably temporally distinct events that may require both unique and overlapping signals. Work in the past several years has started to unravel the signaling networks that control these processes. One of the first pathways identified as important for positive selection was Ras and its downstream effector, the Erk mitogen-activated protein kinase (MAPK) cascade. In this review we examine the factors that connect the TCR to the Ras/Erk cascade in DP thymocytes, as well as what we know about the downstream effectors of the Ras/Erk cascade important for positive selection. We also consider the possible role of this cascade in CD4/CD8 lineage development, and the possible interactions of the Ras/Erk cascade with Notch during these cell fate determination processes.

Expression of the Grb2-related protein of the lymphoid system in B cell subsets enhances B cell antigen receptor signaling through mitogen-activated protein kinase pathways

Adapter proteins play a critical role in regulating signals triggered by Ag receptor cross-linking. These small molecules link receptor proximal events with downstream signaling pathways. In this study, we explore the expression and function of the Grb2-related protein of the lymphoid system (GrpL)/Grb2-related adaptor downstream of Shc adapter protein in human B cells. GrpL is expressed in naive B cells and is down-regulated following B cell Ag receptor ligation. By contrast, germinal center and memory B cells express little or no GrpL. Using human B cell lines, we detected constitutive interactions between GrpL and B cell linker protein, Src homology (SH)2 domain-containing leukocyte protein of 76 kDa, hemopoietic progenitor kinase 1, and c-Cbl. The N-terminal SH3 domain of GrpL binds c-Cbl while the C-terminal SH3 domain binds B cell linker protein and SH2 domain-containing leukocyte protein of 76 kDa. Exogenous expression of GrpL in a GrpL-negative B cell line leads to enhanced Ag receptor-induced extracellular signal-related kinase and p38 mitogen-activated protein kinase phosphorylation. Thus, GrpL expression in human B cell subsets appears to regulate Ag receptor-mediated signaling events.

Loss of neurons in the hippocampus and cerebral cortex of AMSH-deficient mice

AMSH, a molecule that associates with STAM1, is involved in the in vitro cell growth signaling mediated by interleukin 2 and granulocyte-macrophage colony-stimulating factor. To investigate the in vivo functional role of AMSH, we have generated AMSH-deficient mice by gene targeting. The AMSH-deficient mice were morphologically indistinguishable from their littermates at birth, and histopathological examinations revealed normal morphogenesis in all tissues tested. However, all the AMSH-deficient mice exhibited postnatal growth retardation and died between postnatal day 19 (P19) and P23. Examination of brain sections at P6 demonstrated significant loss of neurons and apoptotic cells in the CA1 subfield of the hippocampus. Brain atrophy developed by P16 and was accompanied by complete loss of the CA1 neurons in the hippocampus and marked atrophy of the cerebral cortex. Furthermore, AMSH-deficient hippocampal neuronal cells were unable to survive in vitro, even in the presence of several stimulatory cytokines, while AMSH-deficient cerebellar neurons, thymocytes, and embryonic fibroblasts survived normally. Taken together, these observations indicate that AMSH is an essential molecule for the survival of neuronal cells in early postnatal mice.

Differential requirement for SLP-76 domains in T cell development and function

The hematopoietic cell-specific adaptor protein, SLP-76, is critical for T cell development and mature T cell receptor (TCR) signaling; however, the structural requirements of SLP-76 for mediating thymopoiesis and mature T cell function remain largely unknown. In this study, transgenic mice were generated to examine the requirements for specific domains of SLP-76 in thymocytes and peripheral T cells in vivo. Examination of mice expressing various mutants of SLP-76 on the null background demonstrates a differential requirement for specific domains of SLP-76 in thymocytes and T cells and provides new insight into the molecular mechanisms underlying SLP-76 function.

Positive and negative regulation of T-cell activation by adaptor proteins

Adaptor proteins, molecules that mediate intermolecular interactions, are now known to be as crucial for lymphocyte activation as are receptors and effectors. Extensive work from numerous laboratories has identified and characterized many of these adaptors, demonstrating their roles as both positive and negative regulators. Studies into the molecular basis for the actions of these molecules shows that they function in various ways, including: recruitment of positive or negative regulators into signalling networks, modulation of effector function by allosteric regulation of enzymatic activity, and by targeting other proteins for degradation. This review will focus on a number of adaptors that are important for lymphocyte function and emphasize the various ways in which these proteins carry out their essential roles.

The role of Gads in hematopoietic cell signalling

Gads is a member of the family of SH2 and SH3 domain containing adaptor proteins that is expressed specifically in hematopoietic cells and functions in the coordination of tyrosine kinase mediated signal transduction. Gads plays a critical role in signalling from the T cell receptor by promoting the formation of a complex between SLP-76 and LAT. This complex couples the T cell receptor to Ras through a novel pathway involving PLC-gamma1, Tec family kinases, and RasGRP. Studies with Gads-deficient mice have highlighted its importance for thymocyte proliferation during T cell maturation. Emerging evidence suggests that Gads may also play additional roles in antigen-receptor signalling and receptor tyrosine kinase mediated signalling in other hematopoietic lineages. Gads is a unique member of the Grb2 adaptor family, because its activity can be regulated by caspase cleavage. Gads nucleates multi-protein complexes that are required for tyrosine kinase-dependent signalling in immune cells and may also represent a point of modulation for these pathways through the activation of caspase-dependent signalling events.